Use of Oil-Soluble Surfactants as Breaker Enhancers for VES-Gelled Fluids

ABSTRACT

Fluids viscosified with viscoelastic surfactants (VESs) may have their viscosities reduced (gels broken) by the direct or indirect action of an internal breaker composition that contains at least one mineral oil, at least one polyalphaolefin oil, at least one saturated fatty acid and/or at least one unsaturated fatty acid. The internal breaker may initially be dispersed oil droplets in an internal, discontinuous phase of the fluid. In one non-limiting embodiment, the internal breaker, e.g. mineral oil, is added to the fluid after it has been substantially gelled. An oil-soluble surfactant is present to enhance or accelerate the reduction of viscosity of the gelled aqueous fluid.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No. 11/849,799 filed Sep. 4, 2007 issued Apr. 13, 2010 as U.S. Pat. No. 7,696,135, which is a continuation-in-part of U.S. patent application Ser. No. 11/373,044 filed Mar. 10, 2006 issued Jan. 12, 2010 as U.S. Pat. No. 7,645,724, which in turn claims the benefit of U.S. Provisional Application No. 60/662,336 filed Mar. 16, 2005.

TECHNICAL FIELD

The present invention relates to gelled aqueous treatment fluids used during hydrocarbon recovery operations, and more particularly relates, in one non-limiting embodiment, to methods of “breaking” or reducing the viscosity of aqueous treatment fluids containing viscoelastic surfactant gelling agents used during hydrocarbon recovery operations through internal breakers and separate components that enhance the activity of the internal breakers.

TECHNICAL BACKGROUND

One of the primary applications for viscosified fluids is hydraulic fracturing. Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.

The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates that can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous-based liquids that have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide, for example guar and derivatized guar polysaccharides, is used. The thickened or gelled fluid helps keep the proppants within the fluid. Gelling can be accomplished or improved by the use of crosslinking agents or crosslinkers that promote crosslinking of the polymers together, thereby increasing the viscosity of the fluid. One of the more common crosslinked polymeric fluids is borate crosslinked guar.

The recovery of fracturing fluids may be accomplished by reducing the viscosity of the fluid to a low value so that it may flow naturally from the formation under the influence of formation fluids. Crosslinked gels generally require viscosity breakers to be injected to reduce the viscosity or “break” the gel. Enzymes, oxidizers, and acids are known polymer viscosity breakers. Enzymes are effective within a pH range, typically a 2.0 to 10.0 range, with increasing activity as the pH is lowered towards neutral from a pH of 10.0. Most conventional borate crosslinked fracturing fluids and breakers are designed from a fixed high crosslinked fluid pH value at ambient temperature and/or reservoir temperature. Optimizing the pH for a borate crosslinked gel is important to achieve proper crosslink stability and controlled enzyme breaker activity.

While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles as noted, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, such polymers tend to leave a coating on the proppant and a filter cake of dehydrated polymer on the fracture face even after the gelled fluid is broken. The coating and/or the filter cake may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “microgels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage.

Recently it has been discovered that aqueous drilling and treating fluids may be gelled or have their viscosity increased by the use of non-polymeric viscoelastic surfactants (VES). These VES materials are in many cases advantageous over the use of polymer gelling agents in that they are comprised of low molecular weight surfactants rather than high molecular polymers. The VES materials may leave less gel residue within the pores of oil producing formations, leave no filter cake (dehydrated polymer) on the formation face, leave a minimal amount of residual surfactant coating the proppant, and inherently do not create microgels or “fish-eyes”-type polymeric masses.

However, little progress has been made toward developing internal breaker systems for the non-polymeric VES-based gelled fluids. To this point, VES gelled fluids have relied only on “external” or “reservoir” conditions for viscosity reduction (breaking) and VES fluid removal (clean-up) during hydrocarbon production. Additionally, over the past decade it has been found that reservoir brine dilution has only a minor, if any, breaking effect of VES gel within the reservoir.

Instead, only one reservoir condition is primarily relied on for VES fluid viscosity reduction (gel breaking or thinning), and that has been the rearranging, disturbing, and/or disbanding of the VES worm-like micelle structure by contacting the hydrocarbons within the reservoir, more specifically contacting and mixing with crude oil and condensate hydrocarbons. SPE 30114 describes how reservoir hydrocarbons reduce the viscosity of VES-gelled fluids. SPE 31114 notes that when a VES-gelled fluid contacts crude or condensate reservoir hydrocarbons, the VES-gelled fluid will break, i.e. lose viscosity. SPE 60322 describes how oil or gas reservoir hydrocarbons alter the worm-like micelles of a VES-gelled fluid into spherical micelle structures which results in water-like fluid viscosity. SPE 82245 explains that contact of a VES-gelled fluid system with hydrocarbons causes the fluid to lose its viscosity.

However, in many gas wells and in cases of excessive displacement of crude oil hydrocarbons from the reservoir pores during a VES gel treatment, results have shown many instances where VES fluid in portions of the reservoir are not broken or are incompletely broken resulting in residual formation damage (hydrocarbon production impairment). In such cases post-treatment clean-up fluids composed of either aromatic hydrocarbons, alcohols, surfactants, mutual solvents, and/or other VES breaking additives have been pumped within the VES treated reservoir in order to try and break the VES fluid for removal. However, placement of clean-up fluids is problematic and normally only some sections of the reservoir interval are cleaned up, leaving the remaining sections with unbroken or poorly broken VES gelled fluid that impairs hydrocarbon production. Because of this phenomenon and other occasions where reliance on external factors or mechanisms has failed to clean up the VES fluid from the reservoir during hydrocarbon production, or in cases where the external conditions are slow acting (instances where VES breaking and clean-up takes a long time, such as several days up to possibly months) to break and then produce the VES treatment fluid from the reservoir, and where post-treatment clean-up fluids (i.e. use of external VES breaking solutions) are inadequate in removing unbroken or poorly broken VES fluid from all sections of the hydrocarbon bearing portion of the reservoir, there has been an increasing and important industry need for VES fluids to have internal breakers. Desirable internal breakers that should be developed include breaker systems that use products that are incorporated within the VES-gelled fluid that are activated by downhole temperature that will allow a controlled rate of gel viscosity reduction over a rather short period of time of 1 to 8 hours or so, similar to gel break times common for conventional crosslinked polymeric fluid systems.

A challenge has been that VES-gelled fluids are not comprised of polysaccharide polymers that are easily degraded by use of enzymes or oxidizers, but are comprised of low molecular weight surfactants that associate and form viscous rod- or worm-shaped micelle structures. Conventional enzymes and oxidizers have not been found to act and degrade the surfactant molecules or the viscous micelle structures they form. It is still desirable, however, to provide some mechanism that relies on and uses internal phase breaker products that will help assure complete viscosity break of VES-gelled fluids.

It would be desirable if a viscosity breaking system could be devised to break the viscosity of fracturing and other well completion fluids gelled with and composed of viscoelastic surfactants, particularly break the viscosity completely and relatively quickly.

SUMMARY

There is provided, in one non-limiting form, a method for breaking the viscosity of aqueous fluids gelled with a viscoelastic surfactant (VES). The method includes adding to an aqueous fluid at least one VES in an amount that is effective to increase the viscosity thereof. At least one internal breaker is added to the aqueous fluid before or after the VES is added in an amount effective to reduce the viscosity of the gelled aqueous fluid. The internal breaker may be a mineral oil, a hydrogenated polyalphaolefin oil, a saturated fatty acid, an unsaturated fatty acid and combinations thereof. The mineral oil or hydrogenated polyalphaolefin oil may be advantageously added to the aqueous fluid after it has been substantially gelled by the VES. The method further includes adding to the fluid before, during or after adding the internal breaker is added to the aqueous fluid, at least one oil-soluble surfactant (OSS) breaker enhancer in an amount effective to accelerate reduction of viscosity of the gelled aqueous fluid.

In another non-restrictive embodiment, there is provided an aqueous fluid that includes water and at least one viscoelastic surfactant (VES) in an amount effective to increase the viscosity of the aqueous fluid. The aqueous fluid also includes at least one internal breaker in an amount effective to reduce the viscosity of the gelled aqueous fluid. The internal breaker may again be a mineral oil, a hydrogenated polyalphaolefin oil, a saturated fatty acid, an unsaturated fatty acids and combinations thereof. The aqueous fluid also includes an OSS breaker enhancer in an amount effective to accelerate reduction of viscosity of the gelled aqueous fluid.

The methods and compositions that will be described in further detail below allow the internal breaker to work at lower temperatures in relatively high salinity brine mix waters. High salinity in the aqueous fluids has the effect of slowing the rate of breaking when internal breakers are used in aqueous fluids gelled with a VES. The OSS breaker enhancer overcomes the rate-slowing effect that salinity has on the internal breakers, particularly at lower temperatures. By using these breaker enhancers, internal breakers may now be used at ambient temperatures (e.g. about 80° F. (about 27° C.)) to break VES-gelled fluids having high salinity. This permits the use of a lower amount of the internal breakers.

This may have a number of benefits including, but not necessarily limited to, lowering the overall cost to break a VES-gelled fluid by requiring less internal breaker when an OSS breaker enhancer is present, requiring less internal breaker to achieve a complete viscosity breaker, and/or allowing complete VES-gelled fluid viscosity breaks to be achieved more quickly when using these internal breakers, as compared to an identical method or composition where no breaker enhancer is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the viscosity breaking results as a function of time using four different formulations of an Adriatic seawater brine including 4% KCl, 2.5% WG-3L VES, with varying amounts of a polyenoic acid internal breaker, without and with varying amounts of a transition metal ion source “catalyst” and with and without an OSS breaker enhancer;

FIG. 2 is a graph showing the viscosity breaking results as a function of time using three different formulations of a brine having 9% KCl, 6% WG-3L VES, without and with 5.0 gptg FLC-40L fluid loss control agent, with and without internal breakers, without and with a transition metal ion source “catalyst”, and without and with an OSS breaker enhancer; and

FIG. 3 is a graph showing the viscosity breaking results as a function of time using three different formulations of a brine having 3% KCl, 4% WG-3L VES, without and with polyenoic acid internal breakers, without and with a transition metal ion source “catalyst”, and without and with various OSS breaker enhancers.

DETAILED DESCRIPTION

It has been discovered that oil-soluble surfactants (OSSs) may be used as breaker enhancers for known internal breakers for VES-gelled aqueous fluids, such as the DiamondFRAQ™ technology available from Baker Oil Tools. The oil-soluble surfactants may allow lower internal breaker concentrations to be used to achieve quick and complete VES-gelled fluid breaks (viscosity reductions). This family of oil-soluble surfactants has been discovered to activate polyenoic acids and mineral oil internal breakers so that they may now be used in high salinity brines down to ambient temperatures. This expands and enhances these internal breaking technologies. The oil-soluble surfactants are relatively inexpensive, may be used in relatively low concentrations, and may lower the cost of using the internal breakers.

More specifically, the oil-soluble surfactant breaker enhancers may allow the internal breakers to work at lower temperature in high salinity brine mix waters, in non-limiting examples from about 1% to about 24% by weight (bw) KCl; about 1% to about 44% bw NaBr, about 1% to about 37% bw CaCl₂ brine; about 1% to about 63% bw CaBr₂ brine; etc. These oil-soluble surfactant breaker enhancers may overcome the rate-slowing effect salinity has on internal breakers at lower temperatures, and thus mineral oil-type internal breakers and unsaturated fatty acid-type breakers may now be used at ambient temperatures, such as about 80° F. (about 27° C.), to break VES fluids that have high salinity. By using the oil-soluble surfactant breaker enhancers, lower amounts of the polyenoic acid and/or mineral oil breakers may be used. This will give advantages that include, but are not necessarily limited to, lowering the overall cost to break a VES-gelled aqueous fluid by requiring less internal breaker when an OSS is present, lowering the amount of internal breaker to achieve complete VES viscosity break, allowing a complete VES fluid viscosity break to be achieved more quickly than when using polyenoic or mineral oil breakers alone and combinations of these.

The viscoelastic surfactants are believed to impart viscosity to an aqueous fluid by the molecules organizing themselves into micelles. Spherical micelles do not give increased viscosity, however, when the micelles have an elongated configuration, for instance are “rod-shaped” or “worm-shaped”, they become entangled with one another thereby increasing the viscosity of the fluid.

In one non-limiting embodiment gel-breaking products, such as the internal breakers herein, work by rearrangement of the VES micelle from rod-shaped or worm-shaped elongated structures to spherical structures: that is, the collapse or rearrangement of the viscous elongated micelle structures to non-viscous, more spherical micelle structures. Disaggregating may be understood in one non-limiting embodiment when the micelles are not closely associated physically, that is no longer aggregated or physically interacted together thereby resulting in reduced fluid viscosity, as contrasted with rearrangement which may be understood as a different physical and chemical arrangement or aggregation of the multi-surfactant micelles that has reduced viscosity. However, the inventors do not necessarily want to be limited to any particular mechanism or explanation.

Elongated VES structures may sometimes be referred to as “living” because there is a continuous exchange of VES-type surfactants leaving the VES micelle structures to the aqueous solution and other surfactants leaving the aqueous solution and entering or re-entering the VES micelle structures. It is suspected, in one non-restrictive explanation, that the oil-soluble surfactants over time and under specific conditions become dispersed in the VES elongated micelles and thereby allow a small opening to occur in the VES micelles that allows the internal breakers to enter and/or further destabilize the VES micellar structure, somewhat analogous to a chemical “pinhole” or localized position that opens up or facilitates disruption of the micelles by the internal breakers. However, the inventors do not wish to be limited to any particular explanation or mechanism. With mineral oil internal breakers, particularly the higher molecular weight mineral oils, it appears that the oil-soluble surfactants lower the free energy required to penetrate the VES micelle head groups, and allows or permits, or appears to transport or carry the higher molecular weight mineral oil molecules into the VES micelle. Thus, the ability of the mineral oil internal breaker to penetrate and break the VES structures and to break these structures more quickly is enhanced. In the case of the polyenoic acid internal breakers, it may be that the oil-soluble surfactants hold or retain the auto-oxidized fatty acid compounds (i.e. the hydrophobic-saturated hydrocarbon autooxidation byproducts) to disturb the VES micelle head and/or tail group attractions, thus enhancing the polyenoic auto-oxidation byproduct compound's ability or activity to more readily break the VES micelle structure. Again, it seems to be that the oil-soluble surfactants distribute and create “pinhole” or localized weakened VES head group arrangements or configurations in the VES structure that allows the internal breaking agents to associate and further weaken the “pinhole” locally. In any event, the overall result is that the internal breakers that normally do not work or very slowly work are able to work and/or work more quickly in degrading the VES fluid viscosity when the oil-soluble surfactants are present in the fluid. That is, the breaker enhancers overcome the negative influences of relatively high salinity and relatively low fluid temperature, particularly for high molecular weight type internal breakers; they allow the internal breakers to work more effectively and/or more quickly at lower temperatures, lower concentrations and/or higher mix water salinity. The result is that internal breakers may be used in situations or environments when they normally are ineffective or are effective only at very high concentrations. This discovery expands the domain and use of internal breakers and reduces the cost of using them.

The internal breaker components herein may be added safely and easily to the gel after batch mixing of a VES-gel treatment, or added on-the-fly after continuous mixing of a VES-gel treatment using a liquid additive metering system in one non-limiting embodiment, or the components may be used separately, if needed, as an external breaker solution to remove VES gelled fluids already placed downhole. The mineral oils (being inherently hydrophobic) and/or the mono- and/or polyenoic acids are not solubilized in the brine, but rather interact with the VES surfactant and/or remain as oil droplets to be dispersed and form an emulsion (oil in water type emulsion) and thus there is an oil-stabilized emulsion dispersed in the “internal phase” as a “discontinuous phase” of the brine medium/VES fluid which is the “outer phase” or “continuous phase”. It appears in some cases the mineral oils, e.g., or unsaturated fatty acids (UFAs) are evenly dispersed and are incorporated within the viscous rod- or worm-like shape micelles, however, in other cases, particularly for mineral oils and other saturated hydrocarbons, the oil breaker component can remain as droplets outside of the VES micelles, or as a combination of both micellular locations to various degrees. Rheometer tests have shown, that the incorporation of the UFAs into (within or a part of) the VES micelles does not disturb the viscosity yield of the VES micelles at the levels or amounts of UFAs needed to obtain a complete VES gel viscosity break. This is remarkably unique since oils are considered to readily break the viscosity of VES fluids. The UFAs in particular, have high compatibility with VES fluids until they undergo natural or induced autooxidation, whereby the autooxidation byproducts have been found to readily disturb VES micelles structures and fluid viscosity.

It is surprising and unexpected that mineral oils may serve as internal breakers. This is surprising because the literature teaches that “contact” of a VES-gelled fluid with hydrocarbons, such as those of the formation in a non-limiting example, essentially instantaneously reduces the viscosity of the gel or “breaks” the fluid. By “essentially instantaneously” is meant less than one-half hour. The rate of viscosity break for a given reservoir temperature by the methods described herein is controlled by type and amount of salts within the mix water (i.e. seawater, KCl, NaBr, CaCl₂, CaBr₂, NH₄Cl and the like), presence of a VES gel stabilizer (i.e. MgO, ZnO and the like), presence of a co-surfactant (i.e. sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, potassium laurate, potassium oleate, sodium lauryl phosphate, and the like), VES type (i.e. amine oxide, quaternary ammonium salt, and the like), VES loading, the amount of mineral oil used, the distillation range of the mineral oil, its kinematic viscosity, the presence of components such as aromatic hydrocarbons, and the like. Additionally, as disclosed the rate of viscosity break can be controlled by type and amount of breaker enhancer used.

It is important in most non-limiting embodiments herein to add the lower molecular weight or low viscosity mineral oil products after the aqueous fluid is substantially gelled. Addition of the lower molecular weight mineral oil prior to substantial gelling tends to prevent the gelling or viscosity increase to occur. By “substantially gelled” is meant that at least 90% of the viscosity increase has been achieved before the internal breaker (e.g. mineral oil) is added. Of course, it is acceptable to add the lower molecular weight mineral oil after the gel has completely formed. However, in another non-limiting embodiment, when using the higher molecular weight or higher viscosity mineral oils the order of addition is not important, that is, these type of mineral oils may be added prior to, during, or after the VES product is added to the aqueous fluid and gelled.

Mineral oil (also known as liquid petrolatum) is a by-product in the distillation of petroleum to produce gasoline. It is a chemically inert transparent colorless oil composed mainly of linear, branched, and cyclic alkanes (paraffins) of various molecular weights, related to white petrolatum. Mineral oil is produced in very large quantities, and is thus relatively inexpensive. Mineral oil products are typically highly refined, through distillation, hydrogenation, hydrotreating, and other refining processes, to have improved properties, and the type and amount of refining varies from product to product. Highly refined mineral oil is commonly used as a lubricant and a laxative, and with added fragrance is marketed as “baby oil” in the U.S. Most mineral oil products are very inert and non-toxic, and are commonly used as baby oils and within face, body and hand lotions in the cosmetics industry. Other names for mineral oil include, but are not necessarily limited to, paraffin oil, paraffinic oil, lubricating oil, white mineral oil, and white oil.

In one non-limiting embodiment the mineral oil may be at least 99 wt % paraffinic. Because of the relatively low content of aromatic compounds, mineral oil has a better environmental profile than other oils. In general, the more refined and less aromatic the mineral oil, the better. In another non-restrictive version, the mineral oil may have a distillation temperature range from about 160 to about 550° C., alternatively have a lower limit of about 200° C. and independently an upper limit of about 480° C.; and a kinematic viscosity at 40° C. from about 1 to about 250 cSt, alternatively a lower limit of about 1.2 independently to an upper limit of about 125 cSt. Specific examples of suitable mineral oils include, but are not necessarily limited to, BENOL®, CARNATION®, KAYDOL®, SEMTOL®, HYDROBRITE® and the like mineral oils available from Crompton Corporation, ESCAID®, EXXSOL® ISOPAR® and the like mineral oils available from ExxonMobil Chemical, and similar products from other mineral oil manufacturers. A few non-limiting examples are specified in Table I. The ESCAID 110® and CONOCO LVT-200® mineral oils have been well known components of oil-based drilling muds and the oil industry has considerable experience with these products, thus making them an attractive choice. The white mineral oils from Crompton Corporation with their high purity and high volume use within other industries are also an attractive choice.

TABLE I PROPERTIES OF VARIOUS MINERAL OILS ESCAID EXXSOL HYDROBRITE HYDROBRITE 110 D110 ISOPAR V BENOL 200 1000 Properties Specific Gravity 0.790-0.810 0.780-0.830 0.810-0.830 0.839-0.855 0.845-0.885 0.860-0.885 Viscosity @ 1.3-1.9 — — 18.0-20.0 39.5-46.0 180.0-240.0 40° C. Flash Point (° C.)   77.0 105 118 186 — 288 Pour Point (° C.) — — — −21.0 −9.0 −6.0 Distillation Range IBP (° C.) 200 237 263 — — — Max DP (° C.) 248 277 329 — — — GC Distillation — — — — >380 >407 5% (° C.) Molecular Wt. — — — — — >480 Aromatic <0.5% <1.0% <0.5% — — — Content Note: ESCAID, EXXSOL and ISOPAR are trademarks of ExxonMobil Corporation. BENOL and HYDROBRITE are trademarks of Crompton Corporation.

It has been discovered in breaking VES-gelled fluids prepared in monovalent brines (such as 3% KCl brine) that at temperatures below about 180° F. (82° C.) ESCAID® 110 mineral oil works well in breaking VES-gelled fluids, and that at or above about 140° F. (60° C.) HYDROBRITE® 200 mineral oil works well. The use of mineral oils herein is safe, simple and economical. In some cases for reservoir temperatures between about 120° to about 240° F. (about 49° to about 116° C.) a select ratio of two or more mineral oil products, such as 50 wt % ESCAID® 110 mineral oil to 50 wt % HYDROBRITE® 200 mineral oil may be used to achieve controlled, fast and complete break of a VES-gelled fluid. The use of the breaker enhancers herein expands the ranges of temperatures and brine salt concentrations for which these mineral oils are useful.

It has also been discovered that the type and amount of salt within the mix water used to prepare the VES fluid (such as 3 wt % KCl, 21 wt % CaCl₂, use of natural seawater, and so on) and/or the presence of a VES gel stabilizer (such as VES-STA 1 available from Baker Oil Tools) may affect the activity of a mineral oil in breaking a VES fluid at a given temperature. For example, ESCAID® 110 mineral oil at 5.0 gptg will readily break the 3 wt % KCL based VES fluid at 100° F. (38° C.) over a 5 hour period, and ESCAID® 110 mineral oil may still have utility as a breaker for a 10.0 ppg CaCl₂ (21 wt % CaCl₂) based VES fluid at 250° F. (121° C.).

Other refinery distillates may potentially be used in addition to or alternatively to the mineral oils described herein, as may be hydrocarbon condensation products. Additionally, synthetic mineral oils, such as hydrogenated polyalphaolefins, and other synthetically derived saturated hydrocarbons may be of utility to practice the methods herein. More information about the use of mineral oils, hydrogenated polyalphaolefin oils, and saturated fatty acids as internal breakers may be found in U.S. Patent Application Publication 2007/0056737 A1 published Mar. 15, 2007, incorporated by reference herein in its entirety. In another non-limiting embodiment, natural unsaturated hydrocarbons such as terpenes (e.g. pinene, d-limonene, etc.), saturated fatty acids (e.g. lauric acid, palmitic acid, stearic acid, etc. from plant, fish and/or animal origins) and the like may possibly be used together with or alternatively to the mineral oils herein.

With respect to the unsaturated fatty acid internal breakers such as monoenoic acids and polyenoic acids, as internal breakers, in one non-limiting embodiment these may be specific oils that contain a relatively high amount of either monoenoic or polyenoic acids or both. There are many books and other literature sources that list the multiple types and amounts of fatty acids compositions of oils and fats available from plant, fish, animal, and the like. A polyenoic acid is defined herein as any of various fatty acids having more than one double bond (allyl group) in the carbon chain, e.g. in linoleic acid. Correspondingly, a monoenoic acid is a fatty acid having only one double bond (allyl group). The terms unsaturated fatty acid (UFA) or unsaturated fatty acids (UFAs) are defined herein as oils or fats containing one or the other or both monoenoic and polyenoic fatty acids. Other suitable polyenoic acids include, but are not necessarily limited to omega-3 fatty acids, and omega-6 fatty acids, stearidonic acid, eleostearic acid, eicosadienoic acid, eicosatrienoic acid, arachidonic acid or eicosatetraenoic acid (ETA), eicosapentaenoic acid (EPA), docosapentaenoic acid, docosahexaenoic acid (DHA), cis-linoleic acid, cis-linolenic acid, gamma-linolenic acid, conjugated polyenes, and mixtures thereof. Other suitable monoenoic acids include, but are not necessarily limited to obtusilic acid, caproleic acid, lauroleic acid, linderic acid, myristoleic acid, physeteric acid, tsuzuic acid, palmitoleic acid, petroselinic acid, oleic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, nervonic acid, erucic acid, elaidic acid, t-vaccenic acid, and mixtures thereof.

Oils relatively high in monoenoic and polyenoic acids include, but are not necessarily limited to, flax (linseed) oil, soybean oil, olive oil, canola (rapeseed) oil, chia seed oil, corn oil, cottonseed oil, evening primrose oil, grape seed oil, pumpkin seed oil, safflower oil, sunflower oil, walnut oil, peanut oil, various fish oils, mammal oils, and animal oils or fats and the like.

Any of these oils or fats may be partially hydrogenated, or may contain original or additional preservatives, such as tocopherols, and the like. Additionally any one or more of these oils may be “aged” before use to adjust the product's auto-oxidation activity, along with any one or more reagent or technical grade fatty acids. Allowing a specific fatty acid or UFA oil to “age” allows auto-oxidation to initiate and progress dependant on the amount of time, environmental conditions (temperature, exposure to atmosphere, etc.), presence of other compounds (tocopherols, metal ions, etc), and the like.

It appears that the more double-bonded carbons on the fatty acid carbon chain the more active that fatty acid will be in auto-oxidation, that is, these materials auto-oxidize easier and more quickly. This seems to be a general rule, although other components in the oil may alter this rule. Table II lists the relative rates of oxidation of common fatty acids, from the “Autooxidation” section within “Chemical Reactions of Oil, Fat, and Based Products”, Department of Engineering, Instituto Superior T'echnico, Lisbon, Portugal, October 1997.

TABLE II RELATIVE OXIDATION RATES OF SOME COMMON FATTY ACIDS Total amount of Number of double Relative rate of Fatty acid carbon atoms carbon bonds oxidation Stearic 18 0 1 Oleic 18 1 100 Linoleic 18 2 1200 Linolenic 18 3 2500

Unsaturated fatty acids have been found to break down by “auto-oxidation” into a gamut of VES-breaking products or compositions. Each oil with various monoenoic and polyenoic acids uniquely shows the breakdown of the VES surfactant micelle structure by the presence of these auto-oxidation generated byproducts. Auto-oxidation is also known as autooxidation and lipid peroxidation which includes the oxidation of unsaturated fatty acids. Auto-oxidation in this context may also include a chain reaction—multiple steps and chemical species occur in the oxidative breakdown. Various hydroperoxides may be formed in these auto-oxidations, and end products typically include, but are not necessarily limited to, carbonyl compounds (various aldehydes and ketones), alcohols, acids, and “hydrocarbons” of various types, e.g. alkanes, saturated fatty acids and the like, and mixtures thereof. A variety of technical books and papers list many of the numerous products generated by auto-oxidation (autooxidation) of unsaturated fatty acids.

Fatty acids may also decompose in a water medium and alkaline condition by hydrolysis.

It may be possible that other olefins (e.g. allyl group compounds) may be investigated and employed in the same manner that unsaturated fatty acids have been found to work toward breaking VES-gelled fluids. It also may be possible that mechanisms other than oxidation or hydrolysis may be functioning in generating VES breaking compounds from olefins and olefin derivatives, although the inventors do not want the methods and compositions herein to be limited by any supposed theory.

More information about the use of mono- and polyenoic acids (UFAs) as internal breakers may be found in U.S. Patent Application Publication 2006/0211776 A1 published Sep. 21, 2006, incorporated by reference herein in its entirety.

The breaking or viscosity reduction may be triggered or initiated by heat for both types of internal breakers. The mineral oils and related internal breakers will slowly, upon heating, break or reduce the viscosity of the VES gel with the addition of or in the absence of any other viscosity reducing agent. The use of the oil-soluble enhancers will further influence the triggering and rate of viscosity reduction by both types of internal breakers. In general, the amount of mineral oil needed to break a VES-gelled fluid appears to be temperature dependent, with less needed as the fluid temperature increases, unless one of the breaker enhancers described herein is included. The kinematic viscosity, molecular weight distribution, and amount of impurities (such as aromatics, olefins, and the like) also appear to influence the rate in which a mineral oil will break a VES-gelled fluid at a given temperature. Once a fluid is completely broken a degree of viscosity reheal may occur but in most cases no rehealing is expected because, as noted, it is difficult or impossible to create a gelled fluid in the presence of mineral oil in the first place. An effective amount of mineral oil ranges from about 0.1 to about 15 gptg based on the total fluid, in another non-limiting embodiment from a lower limit of about 0.5. Independently the upper limit of the range may be about 10 gptg based on the total fluid. The necessary proportions or amounts are expected to be lower in the presence of breaker enhancers. (It will be appreciated that units of gallon per thousand gallons (gptg) are readily converted to SI units of the same value as, e.g. liters per thousand liters.)

The mono- and polyenoic acids will slowly to fairly rapidly, upon heating or subjecting the acids to a temperature, auto-oxidize into the VES gel breaking compounds with the addition of or in the absence of any other agent. The amount of altered or oxidized unsaturated fatty acid needed to break a VES-gelled fluid appears to be VES concentration and temperature dependent, with typically more needed as the VES concentration increases and less needed as fluid temperature increases. Of course, as previously discussed, the presence of breaker enhancers will facilitate the reduction of the VES-caused viscosity and lower the temperatures necessary for breaking to occur. Once a fluid is completely broken a degree of viscosity reheal may occur but in most cases no reheal in viscosity will occur and no phase separation of the VES occurs upon fluid cool down, that is when the test fluid is left at test temperature for a sufficient amount of time for complete to near-complete auto-oxidation of the monoenoic and/or polyenoic acids to occur.

The viscosity reduction or breaking action of the previously discussed internal breakers may be enhanced using the oil-soluble surfactants of the present methods and compositions. Suitable oil-soluble surfactant breaker enhancers may include, but are not necessarily limited to, sorbitan fatty acid esters, saponified hard oils, saponified hydrogenated fatty acid oils, long chain alcohols, long chain fatty acid alcohols, long chain fatty amines, long chain sulfates, long chain sulfonates, phospholipids, and lignins. Specific suitable OSSs include, but are not limited to, sorbitan fatty acid esters available from Uniqema as SPAN® 20 sorbitan monolaurate, SPAN® 40 sorbitan monopalmitate, SPAN® 61 sorbitan monostearate, SPAN® 65 sorbitan tristearate, SPAN® 80 sorbitan monooleate, SPAN® 85 sorbitan trioleate, TWEEN® 20 sorbitan monolaurate (with 20 POE (polyoxyethylene) units), TWEEN® 21 sorbitan monolaurate (POE (4)), TWEEN® 40 sorbitan monopalmitate (POE (20)), TWEEN® 60 sorbitan monostearate (POE (20)), TWEEN® 61 sorbitan monostearate (POE (4)), TWEEN® 65 sorbitan tristearate (POE (20)), TWEEN® 80 sorbitan monooleate (POE (20)), TWEEN® 81 sorbitan monooleate (POE (4)), TWEEN® 85 sorbitan trioleate (POE (20)).

Other suitable OSSs include, but are not necessarily limited to, saponified hard oils, which include reactions of saponifying agents like sodium, potassium, calcium, etc. hydroxides with “hard oils” like mahua, tallow, castor, Neem, and the like; saponified hydrogenated fatty acid oils (with the above saponifying agents); partially and/or fully hydrogenated oils like coconut, cottonseed, soybean, and the like; long chain alcohols including long chain linear and branched alcohols that have been ethoxylated and/or propoxylated; long chain fatty amines, including dimeric forms (where two long chain fatty amine surfactants are combined at the head groups); long chain sulfates and sulphonates; and phospholipids, lignins, and mixtures of these.

The proportions of the breaker enhancers useful in the methods and compositions herein range from about 0.001 to about 10% by weight, based on the total VES-gelled fluid composition, and alternatively in one non-limiting embodiment may range from a lower limit of about 0.01, independently to an upper limit of about 2.0% by weight. The amount of OSS needed to enhance the polyenoic breakers, mineral oil breakers and the like internal phase VES breakers will depend on a number of diverse, but interrelated factors including, but not necessarily limited to, type of VES, VES loading, mix water salinity, fluid temperature, the type of OSS, etc. The matrix that the oil-soluble surfactant originates from will also affect the amount to be used, i.e. if the OSS is already within high molecular weight mineral oil before OSS addition to VES fluid, this may slow diffusion of the OSS into VES micelles, etc. The OSS may be added before, during or after addition of the internal breaker to the aqueous fluid gelled with the VES, and as noted, may be combined with the internal breaker prior to addition. In one non-limiting embodiment the oil-soluble surfactant may be advantageously added to high molecular weigh mineral oil, such as ConocoPhillips Pure Performance Base Oil 225N or 600N, at approximately 10 to 20% by volume OSS to 225N or 600N mineral oil.

Controlled viscosity reduction rates using the internal breakers may be achieved at a temperature of from about 70° F. to about 300° F. (about 21 to about 149° C.), and alternatively at a temperature of from about 100° F. independently to an upper end of the range of about 280° F. (about 38 to about 138° C.). The temperature range for a given internal breaker may be shifted lower substantially with the type and amount of the breaker enhancers used herein. It has also been discovered that VES-gelled aqueous fluids containing the small amounts of mineral oils described herein are relatively shear stable and can tolerate some shear before viscosity reduction occurs. In one non-limiting embodiment, the fluid designer would craft the fluid system in such a way that the VES gel would break at or near the expected formation temperature after fracturing was accomplished.

Fluid design may be based primarily on formation temperature, i.e. the temperature the fluid will be heated to naturally in the formation once the treatment is over. Further, fluid design may be based on the expected cool down of the fluid during a treatment. In many cases the fracturing fluid may only experience actual reservoir temperature for 5% to 25% of the job time, and close to 50% of the fluid is never exposed to the original reservoir temperature because of the cool down of the reservoir by the initial fracturing fluid placed into the reservoir. It is because a portion of the fracturing fluid will not see or be exposed to the original reservoir temperature that a cooler temperature is selected that will represent what the fluid will probably see or contact, and thus laboratory break tests are sometimes run at a cooler temperature. There would generally be no additional temperature the VES fluid would see other than original reservoir temperature.

The use of the disclosed internal breaker/breaker enhancer system is ideal for controlling viscosity reduction of VES-based fracturing fluids. The breaking system may also be used for breaking gravel pack fluids, acidizing or near-wellbore clean-up diverter fluids, and loss circulation pill fluids composed of VES. The breaker system may additionally work for foamed fluid applications (hydraulic fracturing, acidizing, and the like), where N₂ or CO₂ gas is used for the gas phase. The VES breaking methods and compositions herein are a significant improvement in that they give breaking rates for VES-based fluids that the industry is accustomed to with conventional polymer based fracturing fluids, such as borate crosslinked guar. Potentially more importantly, in another non-limiting example, the use of the internal breaker/breaker enhancer systems should help assure and improve hydrocarbon production compared to prior art that uses only external mechanisms to break the VES fluid for effective and complete VES fluid clean-up after a treatment.

In one non-limiting embodiment of the invention, the compositions herein will directly degrade the gel created by a VES in an aqueous fluid, and alternatively will reduce the viscosity of the gelled aqueous fluid either directly, or by disaggregation or rearrangement of the VES micellar structure (e.g. collapsing or disturbing the structure). However, the inventors do necessarily not want to be limited to any particular mechanism.

In another non-limiting embodiment, the composition may be modified to slow down or to increase the auto-oxidation of the unsaturated fatty acids. Addition of compounds that influence the rate of auto-oxidation is an important option for the methods and fluids herein, in particular for the lower temperatures to increase the auto-oxidation rate and at higher temperatures to slow down the auto-oxidation rate. Rate control compounds that may be used for slowing down rate of monoenoic and polyenoic acids may be antioxidants such as, but not limited to tocopherol (vitamin E), ascorbic acid (vitamin C), butylated hydroxytoluene (BHT) and other like preservatives, chelants (such as citric acid, phosphates, and EDTA), amino acids, proteins, sugar alcohols (e.g. mannitol, xylitol, lactitol, and sorbitol), salts (such as NaCl, MgCl₂, CaCl₂, NaBr and CaBr₂), and the like. Rate control compounds that may increase the rate of auto-oxidation may be oxidants or pro-oxidants such as, but not limited to persulfate, percarbonate, perbromate, iron, copper, manganese and other transition metals, and the like. It should be noted that there are numerous compounds that may be of utility for regulating the rate of auto-oxidation. The proportion of rate control compounds that may be advantageously used may range from a lower limit of about 0.00001% by weight to an upper limit of about 62% by weight, based on the total weight of fluid, and alternatively from a lower limit of 0.0001% by weight and/or to an upper limit of about 45% by weight. It may be noted that rate controllers used toward the lower limit may be items such as metal ions and rate controllers employed toward the upper limit may be items such as monovalent and/or divalent salts. Chelation of the metal ions tends to slow the rate of auto-oxidation as compared with non-chelated forms of the same metal ions. In one non-limiting understanding, the use of metal ions (whether or not chelated) may be understood as “catalyzing” the auto-oxidation of the UFA.

It is sometimes difficult to specify with accuracy in advance the amount of the various internal breaking components that should be added to a particular aqueous fluid gelled with viscoelastic surfactants to sufficiently or fully break the gel, in general. For instance, a number of factors affect this proportion, including but not necessarily limited to, the particular VES used to gel the fluid; the particular internal breaker used; the particular oil used as a carrier in the case of the unsaturated fatty acids; whether or not a rate-controlling agent is used and what kind; the temperature of the fluid; the downhole pressure of the fluid, the starting pH of the fluid; and the complex interaction of these various factors.

Nevertheless, in order to give an approximate feel for the proportions of the various breaking components to be used in the method of the invention, approximate ranges will be provided. In an alternative, non-limiting embodiment the amount of mineral oil, hydrogenated polyalphaolefin oil and saturated fatty acid that may be effective in the methods and compositions herein may range from about 5 to about 18,000 ppm, based on the total amount of the fluid. In another non-restrictive version herein, the amount of mineral oil may range from a lower end of about 50 independently to an upper end of about 8,000 ppm. The amount of unsaturated fatty acid that may be effective in the methods and compositions may range from about 100 to about 20,000 ppm, based on the total amount of the fluid. In another non-restrictive version, the amount of unsaturated fatty acid may range from a lower limit of about 800 and/or to an upper limit of about 12,000 ppm.

Any suitable mixing apparatus may be used for the methods and fluids herein. In the case of batch mixing, the VES and the aqueous fluid are blended for a period of time sufficient to form a gelled or viscosified solution. The internal breaker, particularly the mineral oil, may advantageously be added after the fluid is formulated or at least after the fluid is substantially gelled. The VES that is useful herein may be any of the VES systems that are familiar to those in the well service industry, and may include, but are not limited to, amines, amine salts, quaternary ammonium salts, amidoamine oxides, amine oxides, mixtures thereof and the like. Suitable amines, amine salts, quaternary ammonium salts, amidoamine oxides, and other surfactants are described in U.S. Pat. Nos. 5,964,295; 5,979,555; and 6,239,183, incorporated herein by reference in their entirety.

Viscoelastic surfactants improve the fracturing (frac) fluid performance through the use of a polymer-free system. These systems, compared to polymeric based fluids, can offer improved viscosity breaking, higher sand trans-port capability, are in many cases more easily recovered after treatment than polymers, and are relatively non-damaging to the reservoir with appropriate contact with sufficient quantity of reservoir hydrocarbons, such as crude oil and condensate. The systems are also more easily mixed “on the fly” in field operations and do not require numerous co-additives in the fluid system, as do some prior systems.

The viscoelastic surfactants suitable for use herein include, but are not necessarily limited to, non-ionic, cationic, amphoteric, and zwitterionic surfactants. Specific examples of zwitterionic/amphoteric surfactants include, but are not necessarily limited to, dihydroxyl alkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkyl amidopropyl betaine and alkylimino mono- or dipropionates derived from certain waxes, fats and oils. Quaternary amine surfactants are typically cationic, and the betaines are typically zwitterionic. The thickening agent may be used in conjunction with an inorganic water-soluble salt or organic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producing formations than cationic fluid types, and are more efficacious per pound than anionic gelling agents. Amine oxide viscoelastic surfactants have the potential to offer more gelling power per pound, making it less expensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂O⁻ may have the following structure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24 carbon atoms and R′ are independently alkyl groups averaging from about 1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl or alkylamido group averaging from about 8 to 16 carbon atoms and R′ are independently alkyl groups averaging from about 2 to 3 carbon atoms. In an alternate, non-restrictive embodiment, the amidoamine oxide gelling agent is Akzo Nobel's Aromox® APA-T formulation, which should be understood as a dipropylamine oxide since both R′ groups are propyl.

Materials sold under U.S. Pat. No. 5,964,295 include CLEARFRAC™, which may also comprise greater than 10% of a glycol. One preferred VES is an amine oxide. As noted, a particularly preferred amine oxide is APA-T, sold by Baker Oil Tools as SURFRAQ™ VES. SURFRAQ is a VES liquid product that is 50% APA-T and greater than 40% propylene glycol. These viscoelastic surfactants are capable of gelling aqueous solutions to form a gelled base fluid. The internal breaker additives herein may be used to prepare a VES system sold by Baker Oil Tools as DIAMONDFRAQ™ fracturing fluid system. DIAMONDFRAQ™ with its assured breaking technology overcomes reliance on external reservoir conditions in order to break, as compared with products such as ClearFRAC™ fracturing fluid system.

The methods and compositions herein also cover commonly known materials as AROMOX® APA-T manufactured by Akzo Nobel and other known viscoelastic surfactant gelling agents common to stimulation treatment of subterranean formations.

The amount of VES included in the fracturing fluid depends on at least two factors. One involves generating enough viscosity to control the rate of fluid leak off into the pores of the fracture, and the second involves creating a viscosity high enough to keep the proppant particles suspended therein during the fluid injecting step, in the non-limiting case of a fracturing fluid. Thus, depending on the application, the VES is added to the aqueous fluid in concentrations ranging from about 0.5 to 25% by volume, alternatively up to about 12 vol % of the total aqueous fluid (from about 5 to 120 gptg). In another non-limiting embodiment, the range for the present formulations is from about 1.0 to about 6.0% by volume VES product. In an alternate, non-restrictive form, the amount of VES ranges from a lower limit of about 2 independently to an upper limit of about 10 volume %.

It is expected that the breaking compositions herein be used to reduce the viscosity of a VES-gelled aqueous fluid regardless of how the VES-gelled fluid is ultimately utilized. For instance, the viscosity breaking compositions could be used in all VES applications including, but not limited to, VES-gelled friction reducers, VES viscosifiers for loss circulation pills, fracturing fluids (including foamed fracturing fluids), gravel pack fluids, viscosifiers used as diverters in acidizing (including foam diverters), VES viscosifiers used to clean up drilling mud filter cake, remedial clean-up of fluids after a VES treatment (post-VES treatment) in regular or foamed fluid forms (i.e. the fluids may be “energized”) with or the gas phase of foam being N₂ or CO₂, and the like.

A value of the methods and compositions herein is that a fracturing or other fluid can be designed to have enhanced breaking characteristics. That is, fluid breaking is no longer dependant on external reservoir conditions for viscosity break and is more controllable: the rate of viscosity reduction, if complete break is achieved/occurs throughout the reservoir interval, and the like.

Importantly, better clean-up of the VES fluid from the fracture and wellbore may be achieved thereby. Better clean-up of the VES directly influences the success of the fracture treatment, which is an enhancement of the well's hydrocarbon productivity. VES fluid clean-up limitations and limitations of the past may be overcome or improved by the use of DiamondFRAQ™ improved VES gel clean-up technology and may now be improved by the use of breaker enhancers.

In order to practice the methods and compositions herein, an aqueous fracturing fluid, as a non-limiting example, is first prepared by blending a VES into an aqueous fluid. The aqueous fluid could be, for example, water, brine, aqueous-based foams or water-alcohol mixtures. Any suitable mixing apparatus may be used for this procedure. In the case of batch mixing, the VES and the aqueous fluid are blended for a period of time sufficient to form a gelled or viscosified solution. As noted, the breaking compositions herein may be added separately after the fluid is substantially gelled, in one non-limiting embodiment. In another non-restricted version, a portion or all of the breaking composition may be added prior to or simultaneously with the VES gelling agent if the breaking agent is in encapsulation form.

Propping agents are typically added to the base fluid after the addition of the VES. Propping agents may include, but are not limited to, for instance, quartz sand grains, glass and ceramic beads, bauxite grains, walnut shell fragments, aluminum pellets, nylon pellets, and the like. The propping agents may be normally used in concentrations between about 1 to 14 pounds per gallon (120-1700 kg/m³) of fracturing fluid composition, but higher or lower concentrations may be used as the fracture design required. The base fluid may also contain other conventional additives common to the well service industry such as water wetting surfactants, non-emulsifiers and the like. As noted herein, the base fluid can also contain other non-conventional additives which can contribute to the breaking action of the VES fluid, and which are added for that purpose in one non-restrictive embodiment.

Any or all of the above mineral oils, hydrogenated polyalphaolefin oils, saturated fatty acids, unsaturated fatty acids and/or breaker enhancers may be provided in an extended release form such as encapsulation by polymer or otherwise, pelletization with binder compounds, absorbed or some other method of layering on a microscopic particle or porous substrate, and/or a combination thereof. Specifically, the internal breakers and breaker enhancers may be micro- and/or macro-encapsulated to permit slow or timed release thereof. In non-limiting examples, the coating material may slowly dissolve or be removed by any conventional mechanism, or the coating could have very small holes or perforations therein for the internal breakers within to diffuse through slowly. For instance, a mixture of fish gelatin and gum acacia encapsulation coating available from ISP Hallcrest, specifically CAPTIVATES® liquid encapsulation technology, may be used to encapsulate the internal breakers and breaker enhancers herein. Also, polymer encapsulation coatings such as used in fertilizer technology available from Scotts Company, specifically POLY-S® product coating technology, or polymer encapsulation coating technology from Fritz Industries could possibly be adapted to the methods herein. The internal breakers may also be absorbed onto zeolites, such as Zeolite A, Zeolite 13X Zeolite DB-2 (available from PQ Corporation, Valley Forge, Pa.) or Zeolites Na-SKS5, Na-SKS6, Na-SKS7, Na-SKS9, Na-SKS10, and Na-SKS13, (available from Hoechst Aktiengesellschaft, now an affiliate of Aventis S.A.), and other porous solid substrates such as MICROSPONGE™ substrates (available from Advanced Polymer Systems, Redwood, Calif.) and cationic exchange materials such as bentonite clay or placed within microscopic particles such as carbon nanotubes or buckminster fullerenes. Further, the internal breakers may be both absorbed into and onto porous or other substrates and then encapsulated or coated, as described above.

In a typical fracturing operation, the fracturing fluid is pumped at a rate sufficient to initiate and propagate a fracture in the formation and to place propping agents into the fracture. A typical fracturing treatment would be conducted by mixing a 20.0 to 60.0 gallon/1000 gal water (volume/volume—the same values may be used with any SI volume unit, e.g. 60.0 liters/1000 liters) amine oxide VES, such as SurFRAQ, in a 2 to 7% (w/v) (166 lb to 581 lb/1000 gal, 19.9 kg to 70.0 kg/m³) KCl solution at a pH ranging from about 6.0 to about 9.0. The breaking components are typically added before or during the VES addition using appropriate mixing and metering equipment, or if needed in a separate step after the fracturing operation is complete or on the fly when going downhole. One unique aspect of the UFA breaking chemistry is how the plant, fish and like type oils may be added and dispersed within the brine mix water prior to the addition of VES, such as the suction side of common hydration units or blender tubs pumps. These oils, used at the typical concentrations needed to achieve quick and complete break, do not initially act as detrimental oils and degrade VES yield and the like. However, most other oils have a detrimental effect to VES yield if already present or when added afterwards. One novelty of the enoic-type oils described herein is they are VES-friendly initially but over time and a given temperature, or in the presence of the breaker enhancers become aggressive VES gel breakers. By “VES-friendly” is meant they are compatible therewith and do not immediate decrease viscosity of aqueous fluids gelled with VES as is seen with many other oils.

In one non-limiting embodiment, the methods and compositions herein are practiced in the absence of gel-forming polymers and/or gels or aqueous fluids having their viscosities enhanced by polymers. However, combination use with polymers and polymer breakers may also be of utility. For instance, polymers may also be added to the VES fluids herein for fluid loss control purposes. Types of polymers that may serve as fluid loss control agents are various starches, polyvinyl acetates, polylactic acid, guar and other polysaccharides, gelatins, and the like.

The present invention will be explained in further detail in the following non-limiting Examples that are only designed to additionally illustrate the invention but not narrow the scope thereof.

General Procedure for Examples

To a blender were added tap water, the indicated proportion of KCl, followed by the indicated proportion of viscoelastic surfactant (WG-3L—AROMOX® APA-T VES available from Akzo Nobel). The blender was used to mix the components on a very slow speed, to prevent foaming, for about 30 minutes to viscosify the VES fluid. Mixed samples were then placed into plastic bottles. Various components singly or together, in various concentrations, were then added to each sample, and the sample was shaken vigorously for 60 seconds. The samples were placed in a water bath at the indicated temperature and visually observed every 30 minutes for viscosity reduction difference between the samples. Since a goal of the research was to find a relatively rapid gel breaking composition, samples were only observed for 24 hours or less.

Viscosity reduction may be visually detected. Shaking the samples and comparing the elasticity of gel and rate of air bubbles rising out of the fluid may be used to estimate the amount of viscosity reduction observed. Measurements using a Brookfield PVS rheometer at the indicated temperatures and pressures at 100 sec⁻¹ were used to acquire quantitative viscosity reduction of each sample.

Formulations 1-4 of FIG. 1

The four formulations which gave the results shown in FIG. 1 had the following compositions:

-   Formulation #1: Adriatic seawater+4% KCl+2.5% WG-3L VES+4.0 gptg     GBW-406L internal breaker (refined soybean oil (polyenoic acid) from     Welch, Holmes & Clark Company) -   Formulation #2: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg     GBW-406L internal breaker+0.4 gptg GBC-4L transition metal ion rate     controller “catalyst” (15% bw CuCl₂.2H₂O solution) -   Formulation #3: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg     GBW-406L internal breaker+2.0 gptg GBC-6L breaker enhancer+0.05 gptg     GBC-4L transition metal ion rate controller “catalyst”. GBC-6L     breaker enhancer is 30% by volume (by) SPAN® 80 sorbitan     monooleate+70% by ConocoPhillips Pure Performance Base (mineral) Oil     225N. -   Formulation #4: Adriatic seawater+4% KCl+2.5% WG-3L VES+3.0 gptg     GBW-406L internal breaker+2.0 gptg GBC-6L breaker enhancer+0.4 gptg     GBC-4L transition metal ion rate controller “catalyst”.

FIG. 1 presents curves for each Formulation showing the viscosity breaking results as a function of time. Formulation 1 which simply had the polyenoic acid breaker (soybean oil) had the most stable viscosity. The addition of the copper chloride solution rate controller which gave Formulation 2 started breaking the viscosity around 16 hours. With the addition of the sorbitan monooleate breaker enhancer herein and mineral oil internal breaker in Formulation 3 (having less of the rate controller), the viscosity broke quickly starting at about 4 hours, where Formulation 4 also having the sorbitan monooleate breaker enhancer herein and mineral oil internal breaker and the same amount of rate controller as in Formulation 2 broke the fastest at about 1 hour. These formulations showed that the breaker enhancers effectively accelerated breaking in high salinity and unique salinity composition mix water.

Formulations of FIG. 2

The aqueous base fluid used in the experiments of FIG. 2 had 9% KCl, 6% WG-3L VES, 5.0 gptg FLC-40L fluid loss control agent (slurried MgO powder mixed in monopropylene glycol available from Baker Oil Tools). The FLC-40L agent was added to the mix water before the WG-3L VES. No breakers were added to the formulation that is the most stable in FIG. 2. In the second formulation the FLC-40L agent, 6 gptg GBW-407L internal breaker, 6.0 gptg GBW-402L internal breaker and 0.3 gptg GBC-4L rate controller were added to the mix water before the WG-3L VES. This formulation gave a noticeable break beginning at about 2 hours. The GBW-407L internal breaker was Fish Oil 18:12TG high EPA and DHA unsaturated fatty acid oil product from Bioriginal Food & Science Corporation. GBW-402L internal breaker was ConocoPhillips Pure Performance Base Oil 225N type mineral oil—a fairly high molecular weight mineral oil. The third formulation was prepared similarly except that 3.0 gptg GBW-402L internal breaker was used along with 1.0 gptg GBC-6L breaker enhancer. This formulation broke the fastest beginning at less than 1 hour further demonstrating that the breaking enhancers of the present methods and compositions are demonstrably effective. It may be noticed how less GBW-402L mineral oil internal breaker was required, but that still faster VES breaking occurs when GBC-6L breaker enhancer is present.

Formulations of FIG. 3

The aqueous base fluid used in the experiments of FIG. 3 had 3% KCl and 4% WG-3L VES. The formulation with no breakers was the most stable with no viscosity decrease in the 12 hours of FIG. 3. A second formulation of the aqueous base fluid with 4 gptg GBW-406L internal breaker and 0.3 gptg GBC-4L transition metal ion rate controller added to the mix water before the WG-3L VES gave the first breaking curve of FIG. 3. The third formulation was similar to the second formulation except that it also included 0.25 gptg GBC-6L breaker enhancer which from the next lowest curve of FIG. 3 shows definite enhancement of breaking the viscosity. In the fourth formulation, the GBC-6L breaker enhancer was replaced by 2.0 gptg Exp-LS90 breaker enhancer (lecithin product EDS90 (phospholipids) from ADM Corporation) which gave a slightly faster break than the third formulation. The fastest viscosity break was given by the fifth formulation which was identical to the third and fourth formulations except that the breaker enhancer was 2.0 gptg GBC-7L breaker enhancer. This breaker enhancer was 20 by volume (bv) SPAN® 85 sorbitan trioleate+80 by ConocoPhillips Pure Performance Base (mineral) Oil 225N internal breaker. For this fifth formulation the initial upper baseline viscosity with no breakers was not reached.

As can be seen, the method of gel breaking described herein is simple, effective, safe, and highly cost-effective. A method is provided for enhancing the breaking the viscosity of aqueous treatment fluids gelled with viscoelastic surfactants (VESs). Compositions and methods are also furnished herein for breaking VES-surfactant fluids controllably, completely and relatively quickly with the breaker enhancers described herein.

Compositions and methods are also disclosed herein for enhancing the breaking of VES-surfactant fluids where contact with reservoir fluids' external breaking mechanism is not required, although in some embodiments heat from the reservoir may help the breaking process. Compositions and methods are additionally provided for breaking VES-surfactant fluids where the breaking additive and breaker enhancer are in a phase internal to the VES-surfactant fluid. Further, methods and VES fluid compositions are described herein for breaking the viscosity of aqueous fluids gelled with viscoelastic surfactants using readily available materials at relatively inexpensive concentrations.

In one non-limiting explanation, it appears the oil-soluble surfactants become dispersed throughout the VES micelles and appear to then be thermodynamically weak points where polyenoic autooxidation products require less thermodynamic energy to disturb the outer layer of “associated hydrophillic head groups and counterions” (K, Ca, etc. counterions) to degrade VES micelle viscosity by micelle rearrangement. This method to degrade VES micelles is not seen as spontaneous or very abrupt but rather a gradual over time mechanism for a given fluid temperature, VES loading, mix water salinity, type and amount of oil-soluble surfactant, and the like, that can be crafted to allow control viscosity break over time, and can give an enhanced viscosity breaking compared to if polyenoic and/or mineral oil breakers alone are used to access, disassociate, and/or disturb the VES micelles and reduce the fluid viscosity.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing methods and compositions for a VES fracturing fluid breaker mechanism. However, it will be evident that various modifications and changes can be made thereto without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, specific combinations of viscoelastic surfactants, internal breakers, breaker enhancers, and other components falling within the claimed parameters, but not specifically identified or tried in a particular composition or fluid, are anticipated to be within the scope of this invention.

The word “comprising” as used throughout the claims is to be interpreted to mean “including but not limited to.” 

1. A method for breaking the viscosity of aqueous fluids gelled with a viscoelastic surfactant (VES) comprising adding to an aqueous fluid at least one VES in an amount effective to increase the viscosity thereof; adding to the aqueous fluid before or after the VES at least one internal breaker in an amount effective to reduce the viscosity of the gelled aqueous fluid, where the internal breaker is selected from the group consisting of mineral oils, hydrogenated polyalphaolefin oils, and saturated fatty acids; and adding to the fluid before, during or after adding the internal breaker, at least one oil-soluble surfactant breaker enhancer in an amount effective to accelerate reduction of viscosity of the gelled aqueous fluid.
 2. The method of claim 1 where the oil-soluble surfactant breaker enhancer is selected from the group consisting of sorbitan fatty acid esters, saponified hard oils, saponified hydrogenated fatty acid oils, long chain alcohols, long chain fatty acid alcohols, long chain fatty amines, long chain sulfates, long chain sulfonates, phospholipids, and lignins.
 3. The method of claim 1 where when the internal breaker is a mineral oil, it is present in an amount from about 5 to about 18,000 ppm based on the total fluid; and the breaker enhancer is present in an amount from about 0.001 to about 10 wt % based on the total fluid.
 4. The method of claim 1 where the internal breaker is a mineral oil and is at least about 99 wt % paraffin.
 5. The method of claim 1 where the internal breaker is a mineral oil and has a distillation temperature in the range from about 160 to about 550° C., and a kinematic viscosity at 40° C. of from about 1 to about 250 cSt.
 6. The method of claim 1 where the internal breaker is present in an oil-soluble internal phase of the aqueous fluid.
 7. The method of claim 1 where the viscosity is broken within about 1 to about 16 hours at a temperature from about 25° C. to about 150° C.
 8. A method for breaking the viscosity of aqueous fluids gelled with a viscoelastic surfactant (VES) comprising adding to an aqueous fluid at least one VES in an amount effective to increase the viscosity thereof; adding to the aqueous fluid before or after the VES at least one internal breaker in an amount effective to reduce the viscosity of the gelled aqueous fluid, where the internal breaker is selected from the group consisting of mineral oils, hydrogenated polyalphaolefin oils, and saturated fatty acids; and adding to the fluid before, during or after adding the internal breaker, from about 0.001 to about 10 wt % based on the total fluid of at least one oil-soluble surfactant breaker enhancer, where the oil-soluble surfactant breaker enhancer is selected from the group consisting of sorbitan fatty acid esters, saponified hard oils, saponified hydrogenated fatty acid oils, long chain alcohols, long chain fatty acid alcohols, long chain fatty amines, long chain sulfates, long chain sulfonates, phospholipids, and lignins.
 9. An aqueous fluid comprising: water; at least one viscoelastic surfactant (VES) in an amount effective to increase the viscosity of the aqueous fluid; at least one internal breaker in an amount effective to reduce the viscosity of the gelled aqueous fluid, where the internal breaker is selected from the group consisting of mineral oils, hydrogenated polyalphaolefin oils, saturated fatty acids, and unsaturated fatty acids; and an oil-soluble surfactant breaker enhancer in an amount effective to accelerate reduction of viscosity of the gelled aqueous fluid.
 10. The aqueous fluid of claim 9 where the oil-soluble surfactant breaker enhancer is selected from the group consisting of sorbitan fatty acid esters, saponified hard oils, saponified hydrogenated fatty acid oils, long chain alcohols, long chain fatty acid alcohols, long chain fatty amines, long chain sulfates, long chain sulfonates, phospholipids, and lignins.
 11. The aqueous fluid of claim 9 where when the internal breaker is a mineral oil, it is present in an amount from about 5 to about 18,000 ppm based on the total fluid, when the internal breaker is an unsaturated fatty acid it is present in an amount from about 100 to about 20,000 ppm based on the total fluid, and the breaker enhancer is present in an amount from about 0.001 to about 10 wt % based on the total fluid.
 12. The aqueous fluid of claim 9 where the internal breaker is a mineral oil and is at least about 99 wt % paraffin.
 13. The aqueous fluid of claim 9 where the internal breaker is a mineral oil and has a distillation temperature in the range from about 160 to about 550° C., and a kinematic viscosity at 40° C. of from about 1 to about 250 cSt.
 14. The aqueous fluid of claim 9 where the internal breaker is selected from the group consisting of: polyenoic acids and monoenoic acids.
 15. The aqueous fluid of claim 14 where the polyenoic acid is selected from the group consisting of linoleic acid, omega-3 fatty acids, omega-6 fatty acids, stearidonic acid, eleostearic acid, eicosadienoic acid, eicosatrienoic acid, arachidonic acid or eicosatetraenoic acid, eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid, cis-linoleic acid, cis-linolenic acid, gamma-linolenic acid, and conjugated polyenes.
 16. The aqueous fluid of claim 14 where the monoenoic acid is selected from the group consisting of obtusilic acid, caproleic acid, lauroleic acid, linderic acid, myristoleic acid, physeteric acid, tsuzuic acid, palmitoleic acid, petroselinic acid, oleic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, nervonic acid, erucic acid, elaidic acid, and t-vaccenic acid.
 17. The aqueous fluid of claim 9 where the internal breaker is present in an oil-soluble internal phase of the aqueous fluid.
 18. An aqueous fluid comprising: water; at least one viscoelastic surfactant (VES) in an amount effective to increase the viscosity of the aqueous fluid; at least one internal breaker in an amount effective to reduce the viscosity of the gelled aqueous fluid, where the internal breaker is selected from the group consisting of mineral oils, hydrogenated polyalphaolefin oils, saturated fatty acids, and unsaturated fatty acids; and from about 0.001 to about 10 wt % based on the total fluid of an oil-soluble surfactant breaker enhancer, where the oil-soluble surfactant breaker enhancer is selected from the group consisting of sorbitan fatty acid esters, saponified hard oils, saponified hydrogenated fatty acid oils, long chain alcohols, long chain fatty acid alcohols, long chain fatty amines, long chain sulfates, long chain sulfonates, phospholipids, and lignins.
 19. The aqueous fluid of claim 18 where the internal breaker is selected from the group consisting of: polyenoic acids and monoenoic acids.
 20. The aqueous fluid of claim 19 where: the polyenoic acid is selected from the group consisting of linoleic acid, omega-3 fatty acids, omega-6 fatty acids, stearidonic acid, eleostearic acid, eicosadienoic acid, eicosatrienoic acid, arachidonic acid or eicosatetraenoic acid, eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid, cis-linoleic acid, cis-linolenic acid, gamma-linolenic acid, and conjugated polyenes, and the monoenoic acid is selected from the group consisting of obtusilic acid, caproleic acid, lauroleic acid, linderic acid, myristoleic acid, physeteric acid, tsuzuic acid, palmitoleic acid, petroselinic acid, oleic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, nervonic acid, erucic acid, elaidic acid, and t-vaccenic acid 